The subject matter of the invention is a process for the production of oligomeric alkylene terephthalates having terminal hydroxyalkylene groups and degrees of condensation of 2 to 20, as well as an apparatus for the performance of this process.
The transesterification of benzene dicarboxylic acid esters with diols is an equilibrium reaction which can be shifted towards the monomeric bis-(hydroxyalkylene)dicarboxylic acid ester by removing the methanol, and further on towards the polyester by the removal of excess diol.
Known processes operate in a discontinuous reaction, for example, in tanks provided with stirrers and other such reactors. The methanol that is released is separated in a superimposed or adjacent rectifying column, and then excess diol is removed in the same manner.
Such discontinuously prepared transesterification products do not fulfill all of the quality requirements in the further polycondensation.
A number of continuously operating apparatus shift the equilibrium of the transesterification reaction in the direction of the desired esters, by means of cascades of stirring tanks, in which case each cascade can have its own rectifying column, although all of the tanks in the cascade can be connected to a common rectifying column. These stirring tank cascades are expensive, they require an individual drive for each reactor and have a large heat radiating surface, and they require a large amount of space.
To avoid these disadvantages, upright columns with a variety of internal fixtures are used, and an additional column is superimposed on them or placed alongside them for the separation of methanol and diol.
For example, it has been proposed to transesterify DMT with ethylene glycol in bubble cap or screen baffle columns or in packed columns. The disadvantage in this case is that higher-boiling by-products that form remain in the sump product, and methanol and low-boiling by-products are given the opportunity to enter reverse reactions and secondary reactions. In German Auslegeschrift 1,593,309, the reaction mixture flows upwardly in a "flooded" column apparatus. By long contact, the vapors that form produce an incomplete transesterification by reverse reaction with methanol vapor.
German Offenlegungsschrift 1,920,954 describes a PG,5 column divided by trays into 20 chambers in which the reaction mixture, by means of tangentially disposed drain tubes, sets the condensation mixture into rotation on the tray below. As the transformation increases, the reaction chambers are exposed to a pressure which diminishes step by step. In this case the necessary mixing action is no longer achieved due to the increased viscosity of the products.
By all of the above-named processes, polyesters of a high degree of condensation are prepared, but not oligomers.